Astronomers Find No Chemical Link Between Stars and the Planets They Host

A team led from the University of Birmingham re-analyzed 45 iron-poor stars and the 64 planets orbiting them and found no statistically significant correlation between a star's chemistry and what its planets are made of. The paper, by Daisy A. Turner and colleagues, has been accepted for publication in Monthly Notices of the Royal Astronomical Society and was posted to arXiv on Sept. 15.

Nobody can sample the inside of a planet in another star system, so models work back from the host star. The two form from the same cloud of gas and dust, and the star's surface chemistry stands in for the raw material the planets were built from. The study tests how far that stand-in holds for stars whose chemistry is nothing like the Sun's.

The team picked hosts that are poor in iron, among the most chemically unlike the Sun. Every star and planet in the sample was then measured the same way: how much of each element the star carries, a fresh mass and radius for the planet, and a model of what the planet is made of all the way through. The 64 planets are super-Earths and sub-Neptunes, sizes between Earth and Neptune that the Solar System does not have.

The authors say the result is not proof that no relation exists. The finding is driven mainly by the large uncertainties on both sides of the comparison, which may be hiding a real relationship, and measuring those uncertainties properly is a step earlier studies have not fully addressed.

A relation is expected on physical grounds, and the paper points to the Solar System, where Earth, Venus and Mars all have magnesium-to-iron ratios close to the Sun's.

Even with sharper measurements a relation may show up only once other factors are brought into the comparison, such as how hot the planet is.